In order to synchronize the timing of a wireless device, such as a user equipment (UE), to the timing of a serving base station, a cell search is performed by the wireless device to locate and synchronize to synchronization signals contained in a downlink transmission from the base station to the wireless device. For example, a long term evolution (LTE) cell search generally consists of the following basic steps:                Acquisition of frequency and symbol synchronization to a cell.        Acquisition of frame timing of the cell—that is, determining the start of the downlink frame.        Determination of the physical-layer cell identity of the cell.        
There are 504 different physical-layer cell identities defined for LTE, where each cell identity corresponds to one specific downlink reference-signal sequence. The set of physical-layer cell identities is further divided into 168 cell-identity groups, with three cell identities within each group. To assist the cell search, two special signals are transmitted on each downlink component carrier: the Primary Synchronization Signal (PSS) and the Secondary Synchronization Signal (SSS). FIGS. 1 and 2 show examples of these signals, namely the PSS 2 and the SSS 4, in relation to a frame 6 for frequency division duplex (FDD) and a frame 8 for time division duplex (TDD).
Shown in FIG. 3 are three PSSs which consist of three Zadoff-Chu (ZC) sequences of length 63, extended with five zeros at the edges and mapped to the center 73 subcarriers, i.e., the center six resource blocks. In particular, an orthogonal frequency division multiplex (OFDM) modulator 12 receives the ZC sequence 10 and modulates the sequence onto the subcarriers. A cyclic-prefix 14 is inserted into the modulated sequences. Note that the center subcarrier is not actually transmitted because it coincides with the DC subcarrier. Thus, only 62 elements of the length-63 ZC sequences are actually transmitted by the base station to the wireless device.
Similar to PSS, the SSS occupies the 72 resource elements, not including the DC carrier, in subframes 0 and 5, for both FDD and TDD. Different synchronization signals can be used by a receiver, separately or jointly, to perform the necessary synchronization and estimation functions. For example, PSS may be more suitable for timing acquisition because of its sequence and correlation properties that allow for an efficient time estimator implementation. On the other hand, SSS is better suited for frequency estimation, possibly jointly with PSS, also owing to its placement within the radio frame.
The SSS should be designed so that:                The two SSS (SSS1 in subframe 0 and SSS2 in subframe 5) take their values from sets of 168 possible values corresponding to the 168 different cell-identity groups.        The set of values applicable for SSS2 is different from the set of values applicable for SSS1 to allow for frame-timing detection from the reception of a single SSS.        
The structure of the two SSSes is illustrated in FIG. 4. SSS1 16 is based on the frequency interleaving of two length-31 m-sequences X and Y, each of which can take 31 different values (actually 31 different shifts of the same m-sequence). Within a cell, SSS2 18 is based on exactly the same two sequences as SSS1 16. However, the two sequences have been swapped in the frequency domain, as shown in FIG. 4. The set of valid combinations of X and Y for SSS1 16 has then been selected so that a swapping of the two sequences in the frequency domain is not a valid combination for SSS1 16. Thus, the above requirements are fulfilled:                The set of valid combinations of X and Y for SSS1 16 (as well as for SSS2 18) are 168, allowing for detection of the physical-layer cell identity.        As the sequences X and Y are swapped between SSS1 16 and SSS2 18, frame timing can be found.        
Traditional communication in terrestrial radio networks is via links between wireless devices, such as UEs, and base stations, such as eNodesB (eNBs) in LTE. However, when two wireless devices are in the vicinity of each other, then direct device to device (D2D) or side link communication is possible. Such communication may be dependent on synchronization information from either a base station or a different node such as a cluster head (CH), i.e., a wireless device acting as synchronization source, providing local synchronization information, or a wireless device enabled to relay synchronization information from a different synchronization source. The synchronization source from a base station or CH is used for intra-cell/cluster communication. The relayed synchronization signal is used for inter-cell/cluster communication. An illustration of synchronization sources from different nodes is shown in FIG. 5.
FIG. 5 shows a communication system 20 with a base station 22 that may service multiple cells and at least one cluster having a cluster head 24 and wireless devices 26. In FIG. 5, the base station 22 or the cluster head 24 may be sources of synchronization signals. For in-coverage D2D scenarios in an LTE system, the synchronization reference is provided by an eNB. The D2D resource pool is signaled by the eNB to indicate the resource used for the D2D communication. For out of coverage D2D scenarios, the synchronization reference is provided by the CH.
The signal design of a device-to-device synchronization signal (D2DSS) is under discussion within bodies forming the third generation partnership project (3GPP). In a current working assumption, a D2DSS comprises at least a primary D2DSS (PD2DSS) and may also include a secondary D2DSS (SD2DSS). Based on this current working assumption, the PD2DSS and SD2DSS use a Zadoff-Chu (ZC) sequence and an M sequence, respectively, which are similar to the LTE PSS and SSS, respectively, and discussed above. Therefore, it is advantageous to reuse the LTE PSS and SSS format for the D2DSS as much as possible in order to reuse the existing timing acquisition circuit to the maximum extent.
An analysis of the peak to average power ratio (PAPR) performance of PSS and SSS shows that the PAPR of SSS is about 2 dB higher than the PABR of PSS. In order to avoid having to transmit the SSS having a higher PAPR, it has been proposed to transmit only a repeated PSS as a D2DSS and avoid transmission of SD2DSS. While this approach effectively solves the PAPR issue, it is observed that pairs of PSS/SSS signals are typically used in existing LTE wireless device implementations in order to obtain frequency synchronization to a given carrier. If SD2DSS is not based on legacy SSS or if SD2DSS is not present at all, as has been proposed, the legacy synchronization algorithms implemented in the devices cannot be fully reused for D2D synchronization. On the other hand, transmitting an SSS with a 2 dB higher PAPR will require more expensive radio amplifiers in the transmitter due to the large signal dynamic range.